1. Field of the Invention
The present invention relates to a predistorter for linearizing a power amplifier for a radio communication transmitter and a predistortion method therefor.
2. Description of the Related Art
Microwave power amplifiers used in base stations or terminals of cellular phone systems require high power efficiency for achieving lower power consumption and compactness. In general, the efficiency of a power amplifier increases as its operating point approaches the saturation output power, and thus, it is desirable that the power amplifier operates in a region close to the saturation output power. However, the power amplifier operating in the region close to the saturation power generates a high level of distortion component. In a base station or a terminal, the power amplifier has to achieve a predetermined attenuation level of a distortion component outside the transmitted signal bandwidth (a predetermined adjacent channel power ratio, for example). Therefore, in order to operate the power amplifier in the region close to the saturation power, the distortion component has to be reduced. In order to reduce the distortion component, researches have been made on nonlinear distortion compensation methods for a distortion generated by the power amplifier.
As a distortion compensation method for a power amplifier, there has been proposed a predistortion method. A predistorter adds a signal to an input signal in advance to cancel a distortion component generated in a power amplifier. The added signal is equal in level and opposite in phase to the distortion component generated in the power amplifier. The amount of distortion compensation by the predistortion method depends on the amplitude and phase error between the added signal and the distortion component. For example, in the case where the input/output characteristics of the power amplifier is represented by a power series model, in order to achieve a distortion compensation of more than 30 dB, the amplitude error and phase error between the signal added by the predistorter and the distortion component generated in the power amplifier have to be suppressed within a range of ±0.28 dB and within a range of ±1.8 degrees, respectively.
FIG. 1 shows a typical configuration of a conventional predistorter based on complex power series representation. In this example, a predistorter 10 predistorts a baseband input signal x(t), the resulting signal r(t) is mixed with a carrier of a frequency fc generated by a carrier generator 32 to produce a radio-frequency-band signal rRF(t), and the radio-frequency-band signal rRF(t) is amplified by a power amplifier 33 for transmission.
The signal x(t) input to the predistorter 10 is divided by a divider 11 and the divided signals are provided to a linear transmission path PTH1 and a distortion generating path PTH2. In the distortion generating path PTH2, a third-order distortion generator 13 and a fifth-order distortion generator 15 generate a third-order distortion signal and a fifth-order distortion signal from the divided input signal x(t), respectively. The vectors of these signals are adjusted by vector adjusters 14 and 16, respectively, and summed up by an adder 18.
On the other hand, the input signal provided to the linear transmission path PTH1 is adjusted in delay time by a delay unit 12. The output from the linear transmission path PTH1 and the output from the distortion generating path PTH2 (that is, the output of the adder 18) are summed up by an adder 19 to produce a predistorted signal, which is output as an output r(t) of the predistorter 10. A distortion detector 34 demodulates the radio-frequency output signal from the power amplifier 33 into a base band signal or intermediate frequency band signal and detects a third-order distortion component and a fifth-order distortion component in the signal. The third-order distortion signal and the fifth-order distortion signal generated by the third-order distortion generator 13 and the fifth-order distortion generator 15 are adjusted by the vector adjusters 14 and 16, respectively, so as to cancel the third-order distortion component and the fifth-order distortion component generated in the power amplifier 33 under the control of a controller 35.
In FIG. 1, in general, the distortion generating path PTH2 comprises a third-order distortion generating path made up of the third-order distortion generator 13 and the vector adjuster 14, a fifth-order distortion generating path made up of the fifth-order distortion generator 15 and the vector adjuster 16 and a (2k−1)th-order distortion generating path made up of a (2k−1)th-order distortion generator and an associated vector adjuster, for example. Here, k denotes an integer equal to or greater than 2.
The (2k−1)th-order distortion generator in the (2k−1)th-order distortion generating path outputs a signal x(t)(2k−1), which is the input signal x(t) to the predistorter 10 raised to the (2k−1)th power (referred to as (2k−1)th order signal, hereinafter). The output signal of the (2k−1)th-order distortion generator has a bandwidth (2k−1) times wider than a bandwidth of the input signal x(t). As shown in FIG. 2, the band of an output signal 2b of the (2k−1)th-order distortion generating path overlaps with the band of an output signal 2a of the linear transmission path PTH1 (shown by the shaded area in FIG. 2). Here, B represent the bandwidth of the input signal. Thus, the output signal of the distortion generating path interferes with the output signal of the linear transmission path. However, if the output signal of the distortion generating path is sufficiently smaller than the output signal of the linear transmission path, the interference can be ignored. However, when the power amplifier operates in the region close to the saturation power to achieve high efficiency, the distortion component increases. Accordingly, the power of the output signal of the distortion generating path has to be increased. Therefore, the interference of the output signal of the distortion generating path with the output signal of the linear transmission path cannot be ignored.
As an example, in the following, there will be described a case where the input signal x(t) to the predistorter 10 is composed of two carriers of equal amplitude. The output r(t) of the predistorter is expressed by the following equation (see the non-patent reference 1):
                              r          ⁡                      (            t            )                          =                              ∑                          k              =              1                        n                    ⁢                                    a                                                2                  ⁢                  k                                -                1                                      ⁢                                                                            x                  ⁡                                      (                    t                    )                                                                                              2                ⁢                                  (                                      k                    -                    1                                    )                                                      ⁢                          x              ⁡                              (                t                )                                                                        (        1        )            
In this equation, the term a1x(t) for k=1 denotes the output signal of the linear transmission path PTH1, and a coefficient a1=α1 denotes a linear gain. The term for k≧2 denotes the output signal of the (2k−1)th order distortion generating path. The gain (α2k−1) and phase (φ2k−1) of the vector adjuster in the (2k−1)th-order distortion generating path are expressed by the following equation;a2k−1=α2k−1ejφ2k−1(k≧2)where x(t) denotes a complex envelope signal input to the predistorter, and r(t) denotes a complex envelope signal output from the predistorter. In the above, the equation is represented using the complex envelope signals. The RF-band signal rRF(t) to be actually transmitted is expressed by the following equation:rRf(t)=Re{r(t)exp(j2πfct)}  (2)In this equation, Re{ } denotes the real part of a complex variable, and fc denotes the frequency of a carrier.
Supposing that the frequency interval between the two carriers of equal amplitude is 2f0, and the amplitude thereof is A, the complex envelope signal x(t) is expressed by the following equation:x(t)=A cos(2πf0t)  (3)From the equation (1), the output signal of the linear transmission path of the predistorter is determined to be:α1A cos(2πf0t)and the output signal of the third-order distortion generating path of the predistorter is determined to be:
            3      4        ⁢          α      3        ⁢          A      3        ⁢          cos      ⁡              (                  2          ⁢          π          ⁢                                          ⁢                      f            0                    ⁢          t                )              ⁢          ⅇ              jφ        3              +            1      4        ⁢          α      3        ⁢          A      3        ⁢          cos      ⁡              (                              3            ·            2                    ⁢          π          ⁢                                          ⁢                      f            0                    ⁢          t                )              ⁢          ⅇ              jφ        3            RF band signal components obtained by up-converting the input signal, the output signal of the linear transmission path and the output signal of the third-order distortion generating path with the carrier frequency fc are shown in FIGS. 3A, 3B and 3C, respectively. In FIGS. 3A to 3D, signals are represented by vectors on a frequency axis. The length and angle of each vector represent the amplitude and phase of the corresponding signal. In FIG. 3A, the input signal is shown as two carrier signals X1L and X1U of equal amplitude. Similarly, in FIG. 3B, the output signal x′(t) of the linear transmission path PTH1 is shown as two carrier signals X1L′ and X1U′ of equal amplitude. In FIG. 3C, the output signal of the third-order distortion generating path is shown as first-order signal components D1L and D1U and third-order signal components D3L and D3U. FIG. 3D shows that sum of the signal components in FIG. 3B and the signal components in FIG. 3C. A signal component X1L″ represents the vector synthesis of the signal components X1L′ and D1L. A signal X1U″ components represents the vector sum of the signal components X1U′ and D1U. The predistorter output r(t), which is made up of sum of the signals from the two paths in the base band by the adder, is expressed by the following equation.
                              r          ⁡                      (            t            )                          =                                            α              1                        ⁢            A            ⁢                                                  ⁢                          cos              ⁡                              (                                  2                  ⁢                  π                  ⁢                                                                          ⁢                                      f                    0                                    ⁢                  t                                )                                              +                                    3              4                        ⁢                          α              3                        ⁢                          A              3                        ⁢                          cos              ⁡                              (                                  2                  ⁢                  π                  ⁢                                                                          ⁢                                      f                    0                                    ⁢                  t                                )                                      ⁢                          ⅇ                              jφ                3                                              +                                    1              4                        ⁢                          α              3                        ⁢                          A              3                        ⁢                          cos              ⁡                              (                                                      3                    ·                    2                                    ⁢                  π                  ⁢                                                                          ⁢                                      f                    0                                    ⁢                  t                                )                                      ⁢                          ⅇ                              jφ                3                                                                        (        4        )            
Up-converting this signal with the carrier frequency fc results in an RF band signal shown in FIG. 3D. As shown in FIG. 3C, the output signal of the third-order distortion generating path contains the signal components D1L and D1U at frequencies fc−f0 and fc+f0, respectively. Due to these signal components, the amplitude and phase of the output signal of the linear transmission path shown in FIG. 3B vary as shown in FIG. 3D. When the power amplifier operates around the saturation region, the variation becomes significant because the level of the output signal of the third-order distortion generating path increases. If such a signal is input to the power amplifier, the amplitude and phase of the distortion component generated in the power amplifier also vary because of the variation of the amplitude and phase of the transmission signal. As a result, to cancel the distortion component generated in the power amplifier, the gain (α3) and phase (φ3) of the vector adjuster in the third-order distortion generating path have to be readjusted considering the above variation. However, such readjustment causes variation of the amplitude and phase of the transmission signal r(t), which causes variation of the amplitude and phase of the generated distortion component. In this way, since the adjustment of the vector adjuster affects not only the output signal of the third-order distortion generating path but also the generated distortion component, adjustment of the vector adjuster becomes complicated.
In the above, only the interference of the output signal of the third-order distortion generating path with the output signal of the linear transmission path has been described. In the case of the output signal of the fifth-order distortion generating path shown in FIG. 4, signal components D1L, D1U, D3L, D3U, D5L and D5U appear at frequencies fc−f0, fc+f0, fc−3f0, fc+3f0, fc−5f0 and fc+5f0, respectively. Therefore, the output signal of the fifth-order distortion generating path interferes not only with the output signal of the linear transmission path but also with the output signal of the third-order distortion generating path.
As can be seen from the above description, the conventional predistorter has a problem that the output signal of the (2k−1)th-order distortion generating path has components that interfere with the output signal of the linear transmission path and the output signals of lower-than-(2k−1)th-order distortion generating paths. While two carriers of equal amplitude have been described as an example, which have discrete spectrum, the same holds true for a signal with continuous spectrum. To solve the problem described above, in the (2k−1)th-order distortion generating path, it is necessary to reduce the components that interfere with the output signal of the linear transmission path and the output signals of lower-than-(2k−1)th-order distortion generating paths.
Non-patent reference: T. Nojima and T. Konno, “Cuber Predistortion Linearizer for Relay Equipment in 800 MHz Band Land Mobile Telephone System”, IEEE Trans. on Vehicular Tech., Vol., VT-34, No. 4, pp. 169-177, November, 1985.